We are interested in determining how a specific site of the messenger RNA (mRNA) containing the initiator codon is selected by the ribosomes in bacterial protein synthesis. We intend to study the problem by kinetic analysis of the binding of the initiator tRNA (fMet-tRNA) to E.coli ribosomes with limiting concentrations of MS2 viral RNA and various synthetic mRNAs. Under such conditions, the binding reaction is pseudo-first-order with respect to mRNA concentration, and the rate constants of the reaction reflect the efficiency of the mRNAs. If the rate constants with the synthetic mRNAs, which are unlikely to be specifically recognized by the ribosomes or initiation factors, are comparable to or greater than the rate constant obtained with MS2 RNA, the results would imply that no special initiation signal in the mRNA is obligatorily recognized in initiator site selection. We can thus conclude with more assurance either that the initiator site is selected because it is specifically recognized by the ribosomes and/or by initiation factor IF-3, as is generally believed, or that the site is selected without specific recognition because it is uniquely accessible to the ribosomes, as we have suggested. As secondary objectives, we will study the basis for the difference in the translational specificity of E. coli and B. stearothermophilus ribosomes and for the activation of E. coli ribosomes by the supernatant factors EF-Ts and EF-G. We will also examine the specificity of rabbit reticulocyte ribosomes and initiation factors, as a preliminary step to the investigation of the regulation of eukaryotic protein synthesis, by kinetic analysis similar to that described above. All of these studies should help to resolve the very important problem of how one of the central processes in the cell is initiated.